Birth of ophthalmic trigeminal neurons initiates early in the placodal ectoderm.

The largest of the cranial ganglia, the trigeminal ganglion, relays cutaneous sensations of the head to the central nervous system. Its sensory neurons have a dual origin from both ectodermal placodes and neural crest. Here, we show that the birth of neurons derived from the chick ophthalmic trigeminal placode begins prior to their ingression (HH11), as early as HH8, and considerably earlier than previously suspected (HH16). Furthermore, cells exiting the cell cycle shortly thereafter express the ophthalmic trigeminal placode marker Pax3 (HH9). At HH11, these postmitotic Pax3+ placode cells begin to express the pan-neuronal marker neurofilament while still in the ectoderm. Analysis of the ectodermal origin and distribution of these early postmitotic neurons reveals that the ophthalmic placode extends further rostrally than anticipated, contributing to neurons that reside in and make a significant contribution to the ophthalmic trigeminal nerve. These data redefine the timing and extent of neuron formation from the ophthalmic trigeminal placode.